Transversely-excited film bulk acoustic resonator using pre-formed cavities

ABSTRACT

Acoustic resonator devices, filter devices, and methods of making acoustic resonator devices and filter devices. An acoustic resonator device includes a substrate with a cavity and an alignment pattern in a surface of the substrate. The cavity and the alignment pattern have a same depth. A back surface of a piezoelectric plate is attached to the surface of the substrate. A portion of the piezoelectric plate that spans the cavity forms a diaphragm. An interdigital transducer (IDT) is on a front surface of the piezoelectric plate. Interleaved fingers of the IDT are on the diaphragm.

RELATED APPLICATION INFORMATION

This patent is a continuation of application Ser. No. 16/920,129, filed Jul. 2, 2020, entitled TRANSVERSELY-EXCITED FILM BULK ACOUSTIC RESONATOR USING PRE-FORMED CAVITIES, which claims priority from provisional patent application No. 62/873,726, entitled TRANSVERSELY-EXCITED BULK ACOUSTIC RESONATORS USING PRE-FORMED CAVITIES, filed Jul. 12, 2019, and provisional application No. 62/873,732, entitled METHOD TO FABRICATE XBAR WITH PRE-FORMED CAVITIES, filed Jul. 12, 2019, the entire contents of both of which are incorporated herein by reference.

Application Ser. No. 16/920,129 is also a continuation-in-part of application Ser. No. 16/438,121, now patent U.S. Pat. No. 10,756,697 B2, filed Jun. 11, 2019, entitled TRANSVERSELY-EXCITED FILM BULK ACOUSTIC RESONATOR, which is a continuation-in-part of application Ser. No. 16/230,443, now patent U.S. Pat. No. 10,491,192 B2, filed Dec. 21, 2018, entitled TRANSVERSELY-EXCITED FILM BULK ACOUSTIC RESONATOR, which claims priority from the following provisional patent applications: application 62/685,825, filed Jun. 15, 2018, entitled SHEAR-MODE FBAR (XBAR); application 62/701,363, filed Jul. 20, 2018, entitled SHEAR-MODE FBAR (XBAR); application 62/741,702, filed Oct. 5, 2018, entitled 5 GHZ LATERALLY-EXCITED BULK WAVE RESONATOR (XBAR); application 62/748,883, filed Oct. 22, 2018, entitled SHEAR-MODE FILM BULK ACOUSTIC RESONATOR; and application 62/753,815, filed Oct. 31, 2018, entitled LITHIUM TANTALATE SHEAR-MODE FILM BULK ACOUSTIC RESONATOR.

NOTICE OF COPYRIGHTS AND TRADE DRESS

A portion of the disclosure of this patent document contains material which is subject to copyright protection. This patent document may show and/or describe matter which is or may become trade dress of the owner. The copyright and trade dress owner has no objection to the facsimile reproduction by anyone of the patent disclosure as it appears in the Patent and Trademark Office patent files or records, but otherwise reserves all copyright and trade dress rights whatsoever.

BACKGROUND Field

This disclosure relates to radio frequency filters using acoustic wave resonators, and specifically to filters for use in communications equipment.

Description of the Related Art

A radio frequency (RF) filter is a two-port device configured to pass some frequencies and to stop other frequencies, where “pass” means transmit with relatively low signal loss and “stop” means block or substantially attenuate. The range of frequencies passed by a filter is referred to as the “pass-band” of the filter. The range of frequencies stopped by such a filter is referred to as the “stop-band” of the filter. A typical RF filter has at least one pass-band and at least one stop-band. Specific requirements on a pass-band or stop-band depend on the specific application. For example, a “pass-band” may be defined as a frequency range where the insertion loss of a filter is better than a defined value such as 1 dB, 2 dB, or 3 dB. A “stop-band” may be defined as a frequency range where the rejection of a filter is greater than a defined value such as 20 dB, 30 dB, 40 dB, or greater depending on application.

RF filters are used in communications systems where information is transmitted over wireless links. For example, RF filters may be found in the RF front-ends of cellular base stations, mobile telephone and computing devices, satellite transceivers and ground stations, IoT (Internet of Things) devices, laptop computers and tablets, fixed point radio links, and other communications systems. RF filters are also used in radar and electronic and information warfare systems.

RF filters typically require many design trade-offs to achieve, for each specific application, the best compromise between performance parameters such as insertion loss, rejection, isolation, power handling, linearity, size and cost. Specific design and manufacturing methods and enhancements can benefit simultaneously one or several of these requirements.

Performance enhancements to the RF filters in a wireless system can have broad impact to system performance. Improvements in RF filters can be leveraged to provide system performance improvements such as larger cell size, longer battery life, higher data rates, greater network capacity, lower cost, enhanced security, higher reliability, etc. These improvements can be realized at many levels of the wireless system both separately and in combination, for example at the RF module, RF transceiver, mobile or fixed sub-system, or network levels.

The desire for wider communication channel bandwidths will inevitably lead to the use of higher frequency communications bands. The current LTE™ (Long Term Evolution) specification defines frequency bands from 3.3 GHz to 5.9 GHz. Some of these bands are not presently used. Future proposals for wireless communications include millimeter wave communication bands with frequencies up to 28 GHz.

High performance RF filters for present communication systems commonly incorporate acoustic wave resonators including surface acoustic wave (SAW) resonators, bulk acoustic wave (BAW) resonators, film bulk acoustic wave resonators (FBAR), and other types of acoustic resonators. However, these existing technologies are not well-suited for use at the higher frequencies proposed for future communications networks.

DESCRIPTION OF THE DRAWINGS

FIG. 1 includes a schematic plan view and two schematic cross-sectional views of a transversely-excited film bulk acoustic resonator (XBAR).

FIG. 2 is an expanded schematic cross-sectional view of a portion of the XBAR of FIG. 1.

FIG. 3A is an alternative schematic cross-sectional view of the XBAR of FIG. 1.

FIG. 3B is another alternative schematic cross-sectional view of the XBAR of FIG. 1.

FIG. 3C is an alternative schematic plan view of an XBAR

FIG. 4 is a graphic illustrating a shear horizontal acoustic mode in an XBAR.

FIG. 5A is a schematic cross-sectional view of a substrate for an XBAR.

FIG. 5B is a schematic cross-sectional view of a piezoelectric substrate for forming a piezoelectric plate of an XBAR.

FIG. 5C is a schematic cross-sectional view of the substrate of FIG. 5A bonded to the piezoelectric substrate of FIG. 5B.

FIG. 5D is a schematic cross-sectional view of the device of FIG. 5C after the piezoelectric plate has been separated from the piezoelectric substrate.

FIG. 5E is a schematic cross-sectional view of the device of FIG. 5D after interdigital transducer formation.

FIG. 6 is a schematic circuit diagram and layout of a filter using XBARs.

FIG. 7 is a flow chart of a process for fabricating an XBAR.

Throughout this description, elements appearing in figures are assigned three-digit or four-digit reference designators, where the two least significant digits are specific to the element and the one or two most significant digit is the figure number where the element is first introduced. An element that is not described in conjunction with a figure may be presumed to have the same characteristics and function as a previously-described element having the same reference designator.

DETAILED DESCRIPTION

Description of Apparatus

FIG. 1 shows a simplified schematic top view and orthogonal cross-sectional views of a transversely-excited film bulk acoustic resonator (XBAR) 100. XBAR resonators such as the resonator 100 may be used in a variety of RF filters including band-reject filters, band-pass filters, duplexers, and multiplexers. XBARs are particularly suited for use in filters for communications bands with frequencies above 3 GHz.

The XBAR 100 is made up of a thin film conductor pattern formed on a surface of a piezoelectric plate 110 having parallel front and back surfaces 112, 114, respectively. The piezoelectric plate is a thin single-crystal layer of a piezoelectric material such as lithium niobate, lithium tantalate, lanthanum gallium silicate, gallium nitride, or aluminum nitride. The piezoelectric plate is cut such that the orientation of the X, Y, and Z crystalline axes with respect to the front and back surfaces is known and consistent. In the examples presented in this patent, the piezoelectric plates are Z-cut, which is to say the Z axis is normal to the front and back surfaces 112, 114. However, XBARs may be fabricated on piezoelectric plates with other crystallographic orientations.

The back surface 114 of the piezoelectric plate 110 is attached to a surface of the substrate 120 except for a portion of the piezoelectric plate 110 that forms a diaphragm 115 spanning a cavity 140 formed in the substrate. The portion of the piezoelectric plate that spans the cavity is referred to herein as the “diaphragm” 115 due to its physical resemblance to the diaphragm of a microphone. As shown in FIG. 1, the diaphragm 115 is contiguous with the rest of the piezoelectric plate 110 around all of a perimeter 145 of the cavity 140. In this context, “contiguous” means “continuously connected without any intervening item”.

The substrate 120 provides mechanical support to the piezoelectric plate 110. The substrate 120 may be, for example, silicon, sapphire, quartz, or some other material or combination of materials. The back surface 114 of the piezoelectric plate 110 may be bonded to the substrate 120 using a wafer bonding process. Alternatively, the piezoelectric plate 110 may be grown on the substrate 120 or attached to the substrate in some other manner. The piezoelectric plate 110 may be attached directly to the substrate or may be attached to the substrate 120 via one or more intermediate material layers.

“Cavity” has its conventional meaning of “an empty space within a solid body.” The cavity 140 may be a hole completely through the substrate 120 (as shown in Section A-A and Section B-B) or a recess in the substrate 120 (as shown subsequently in FIG. 3A and FIG. 3B). The cavity 140 may be formed, for example, by selective etching of the substrate 120 before or after the piezoelectric plate 110 and the substrate 120 are attached.

The conductor pattern of the XBAR 100 includes an interdigital transducer (IDT) 130. The IDT 130 includes a first plurality of parallel fingers, such as finger 136, extending from a first busbar 132 and a second plurality of fingers extending from a second busbar 134. The first and second pluralities of parallel fingers are interleaved. The interleaved fingers overlap for a distance AP, commonly referred to as the “aperture” of the IDT. The center-to-center distance L between the outermost fingers of the IDT 130 is the “length” of the IDT.

The first and second busbars 132, 134 serve as the terminals of the XBAR 100. A radio frequency or microwave signal applied between the two busbars 132, 134 of the IDT 130 excites a primary acoustic mode within the piezoelectric plate 110. As will be discussed in further detail, the primary acoustic mode is a bulk shear mode where acoustic energy propagates along a direction substantially orthogonal to the surface of the piezoelectric plate 110, which is also normal, or transverse, to the direction of the electric field created by the IDT fingers. Thus, the XBAR is considered a transversely-excited film bulk wave resonator.

The IDT 130 is positioned on the piezoelectric plate 110 such that at least the fingers of the IDT 130 are disposed on the portion 115 of the piezoelectric plate that spans, or is suspended over, the cavity 140. As shown in FIG. 1, the cavity 140 has a rectangular shape with an extent greater than the aperture AP and length L of the IDT 130. A cavity of an XBAR may have a different shape, such as a regular or irregular polygon. The cavity of an XBAR may more or fewer than four sides, which may be straight or curved.

For ease of presentation in FIG. 1, the geometric pitch and width of the IDT fingers is greatly exaggerated with respect to the length (dimension L) and aperture (dimension AP) of the XBAR. A typical XBAR has more than ten parallel fingers in the IDT 110. An XBAR may have hundreds, possibly thousands, of parallel fingers in the IDT 110. Similarly, the thickness of the fingers in the cross-sectional views is greatly exaggerated.

FIG. 2 shows a detailed schematic cross-sectional view of the XBAR 100. The piezoelectric plate 110 is a single-crystal layer of piezoelectrical material having a thickness ts. ts may be, for example, 100 nm to 1500 nm. When used in filters for LTE™ bands from 3.4 GHZ to 6 GHz (e.g. bands 42, 43, 46), the thickness ts may be, for example, 200 nm to 1000 nm.

A front-side dielectric layer 214 may optionally be formed on the front side of the piezoelectric plate 110. The “front side” of the XBAR is, by definition, the surface facing away from the substrate. The front-side dielectric layer 214 has a thickness tfd. The front-side dielectric layer 214 is formed between the IDT fingers 238. Although not shown in FIG. 2, the front side dielectric layer 214 may also be deposited over the IDT fingers 238. A back-side dielectric layer 216 may optionally be formed on the back side of the piezoelectric plate 110. The back-side dielectric layer 216 has a thickness tbd. The front-side and back-side dielectric layers 214, 216 may be a non-piezoelectric dielectric material, such as silicon dioxide or silicon nitride. tfd and tbd may be, for example, 0 to 500 nm. tfd and tbd are typically less than the thickness ts of the piezoelectric plate. tfd and tbd are not necessarily equal, and the front-side and back-side dielectric layers 214, 216 are not necessarily the same material. Either or both of the front-side and back-side dielectric layers 214, 216 may be formed of multiple layers of two or more materials.

The IDT fingers 238 may be aluminum, a substantially aluminum alloys, copper, a substantially copper alloys, beryllium, gold, or some other conductive material. Thin (relative to the total thickness of the conductors) layers of other metals, such as chromium or titanium, may be formed under and/or over the fingers to improve adhesion between the fingers and the piezoelectric plate 110 and/or to passivate or encapsulate the fingers. The busbars (132, 134 in FIG. 1) of the IDT may be made of the same or different materials as the fingers.

Dimension p is the center-to-center spacing or “pitch” of the IDT fingers, which may be referred to as the pitch of the IDT and/or the pitch of the XBAR. Dimension w is the width or “mark” of the IDT fingers. The IDT of an XBAR differs substantially from the IDTs used in surface acoustic wave (SAW) resonators. In a SAW resonator, the pitch of the IDT is one-half of the acoustic wavelength at the resonance frequency. Additionally, the mark-to-pitch ratio of a SAW resonator IDT is typically close to 0.5 (i.e. the mark or finger width is about one-fourth of the acoustic wavelength at resonance). In an XBAR, the pitch p of the IDT is typically 2 to 20 times the width w of the fingers. In addition, the pitch p of the IDT is typically 2 to 20 times the thickness is of the piezoelectric slab 212. The width of the IDT fingers in an XBAR is not constrained to one-fourth of the acoustic wavelength at resonance. For example, the width of XBAR IDT fingers may be 500 nm or greater, such that the IDT can be fabricated using optical lithography. The thickness tm of the IDT fingers may be from 100 nm to about equal to the width w. The thickness of the busbars (132, 134 in FIG. 1) of the IDT may be the same as, or greater than, the thickness tm of the IDT fingers.

FIG. 3A and FIG. 3B show two alternative cross-sectional views along the section plane A-A defined in FIG. 1. In FIG. 3A, a piezoelectric plate 310 is attached to a substrate 320. A portion of the piezoelectric plate 310 forms a diaphragm 315 spanning a cavity 340 in the substrate. The cavity 340 does not fully penetrate the substrate 320. Fingers of an IDT are disposed on the diaphragm 315. The cavity 340 may be formed, for example, by etching the substrate 320 before attaching the piezoelectric plate 310. Alternatively, the cavity 340 may be formed by etching the substrate 320 with a selective etchant that reaches the substrate through one or more openings (not shown) provided in the piezoelectric plate 310. In this case, the diaphragm 315 may contiguous with the rest of the piezoelectric plate 310 around a large portion of a perimeter 345 of the cavity 340. For example, the diaphragm 315 may contiguous with the rest of the piezoelectric plate 310 around at least 50% of the perimeter 345 of the cavity 340.

In FIG. 3B, the substrate 320 includes a base 322 and an intermediate layer 324 disposed between the piezoelectric plate 310 and the base 322. For example, the base 322 may be silicon and the intermediate layer 324 may be silicon dioxide or silicon nitride or some other material. A portion of the piezoelectric plate 310 forms a diaphragm 315 spanning a cavity 340 in the intermediate layer 324. Fingers of an IDT are disposed on the diaphragm 315. The cavity 340 may be formed, for example, by etching the intermediate layer 324 before attaching the piezoelectric plate 310. Alternatively, the cavity 340 may be formed by etching the intermediate layer 324 with a selective etchant that reaches the substrate through one or more openings provided in the piezoelectric plate 310. In this case, the diaphragm 315 may contiguous with the rest of the piezoelectric plate 310 around a large portion of a perimeter 345 of the cavity 340. For example, the diaphragm 315 may contiguous with the rest of the piezoelectric plate 310 around at least 50% of the perimeter 345 of the cavity 340. Although not shown in FIG. 3B, a cavity formed in the intermediate layer 324 may extend into the base 322.

FIG. 3C is a schematic plan view of another XBAR 350. The XBAR 350 includes an IDT formed on a piezoelectric plate 310. A portion of the piezoelectric plate 310 forms a diaphragm spanning a cavity in a substrate. In this example, the perimeter 345 of the cavity has an irregular polygon shape such that none of the edges of the cavity are parallel, nor are they parallel to the conductors of the IDT. A cavity may have a different shape with straight or curved edges.

FIG. 4 is a graphical illustration of the primary acoustic mode of interest in an XBAR. FIG. 4 shows a small portion of an XBAR 400 including a piezoelectric plate 410 and three interleaved IDT fingers 430. An RF voltage is applied to the interleaved fingers 430. This voltage creates a time-varying electric field between the fingers. The direction of the electric field is lateral, or parallel to the surface of the piezoelectric plate 410, as indicated by the arrows labeled “electric field”. Due to the high dielectric constant of the piezoelectric plate, the electric field is highly concentrated in the plate relative to the air. The lateral electric field introduces shear deformation, and thus strongly excites a shear-mode acoustic mode, in the piezoelectric plate 410. In this context, “shear deformation” is defined as deformation in which parallel planes in a material remain parallel and maintain a constant distance while translating relative to each other. A “shear acoustic mode” is defined as an acoustic vibration mode in a medium that results in shear deformation of the medium. The shear deformations in the XBAR 400 are represented by the curves 460, with the adjacent small arrows providing a schematic indication of the direction and magnitude of atomic motion. The degree of atomic motion, as well as the thickness of the piezoelectric plate 410, have been greatly exaggerated for ease of visualization. While the atomic motions are predominantly lateral (i.e. horizontal as shown in FIG. 4), the direction of acoustic energy flow of the excited primary shear acoustic mode is substantially orthogonal to the surface of the piezoelectric plate, as indicated by the arrow 465.

Considering FIG. 4, there is essentially no electric field immediately under the IDT fingers 430, and thus acoustic modes are only minimally excited in the regions 470 under the fingers. There may be evanescent acoustic motions in these regions. Since acoustic vibrations are not excited under the IDT fingers 430, the acoustic energy coupled to the IDT fingers 430 is low (for example compared to the fingers of an IDT in a SAW resonator), which minimizes viscous losses in the IDT fingers.

An acoustic resonator based on shear acoustic wave resonances can achieve better performance than current state-of-the art film-bulk-acoustic-resonators (FBAR) and solidly-mounted-resonator bulk-acoustic-wave (SMR BAW) devices where the electric field is applied in the thickness direction. In such devices, the acoustic mode is compressive with atomic motions and the direction of acoustic energy flow in the thickness direction. In addition, the piezoelectric coupling for shear wave XBAR resonances can be high (>20%) compared to other acoustic resonators. Thus, high piezoelectric coupling enables the design and implementation of microwave and millimeter-wave filters with appreciable bandwidth.

Pre-Formed Cavities

XBARs may be divided into two broad categories known as “the swimming pool option” and the “backside etch option”. With the swimming pool option, the piezoelectric plate is attached to a substrate and the active portion of the piezoelectric plate floats over a cavity (the “swimming pool”) formed in the substrate, as shown in FIG. 3B and FIG. 3C. With the backside etch option, the piezoelectric plate is attached to a substrate and the active portion of the piezoelectric plate floats over a void etched through the substrate from the back side (i.e. the side opposite the piezoelectric plate), as shown in the cross-sectional views of FIG. 1.

FIG. 5A is a view of a substrate 520 for an XBAR with a “swimming pool” cavity. The substrate 520 can be formed of a suitable material as described above, such as silicon, sapphire, quartz, or some other material or combination of materials. The substrate 520 has one or more cavities 540 (or trenches) pre-formed into the substrate 520, meaning that the cavities are formed prior to the formation of the other parts of the device. These cavities 540 can be formed by any suitable method, such as etching with a selective etchant. Though four cavities are shown in FIG. 5A, more or fewer cavities can be formed in the substrate.

Alignment patterns 542 can also be formed in the substrate 520. In FIG. 5A, the alignment patterns 542 are shown at the perimeter of the substrate 520, but the alignment patterns can be formed at any suitable location on the substrate that does not interfere with the cavities, such as at certain edges or between cavities. The alignment patterns may be, for example, reticles or other shapes suitable for aligning photomasks used in subsequent process steps.

The cavities 540 and the alignment patterns 542 are shown as trenches with square shaped cross-sections, but these features can have any suitable shape, such as trenches with slanted sides and slanted or rounded bottom surfaces. For example, the alignment patterns can be etched features defined by the same mask used to define the cavities, such that the alignment patterns and cavities can be formed in a single process sequence. The alignment patterns can then facilitate alignment of photomasks used to pattern the metal layers such that the IDT fingers are positioned over the cavities.

The substrate 520 can also be coated with SiO2, or another suitable coating, on a surface of the substrate that will later be bonded with a piezoelectric plate. The surfaces of the cavities (i.e., the bottoms and sides of the cavities) may or may not be coated. Other surfaces of the substrate can be coated with the same or different materials, or left uncoated.

FIG. 5B is a view of a piezoelectric slab 508 for forming a piezoelectric plate 510 for an XBAR. The piezoelectric slab 508 can be formed of any suitable material as described above, such as LiNbO3 and LiTaO3. Ions may be implanted into the back surface 511 of the piezoelectric slab 508. The energy of the ion implantation determines the depth to which the ions are implanted. The implanted ions create defects in the crystalline structure along the dashed line 509, which facilitates the wafer splitting. Thin piezoelectric slabs can also be produced by the method of wafer polishing, in addition to ‘ion-slicing’.

FIG. 5C is a schematic plan cross-sectional view of the substrate 520 of FIG. 5A bonded to the piezoelectric slab 508 of FIG. 5B. The piezoelectric slab 508 is bonded to the substrate 520 using a wafer bonding process. The piezoelectric slab 508 may be aligned to the alignment pattern of the substrate 520.

A piezoelectric plate 510 can then be separated from the piezoelectric slab 508 at dashed line 509, as shown in FIG. 5D. In the ion-slice method, thermal shock or another suitable technique is used to fracture the piezoelectric slab 508 along the defect plane at dashed line 509, leaving the piezoelectric plate 510 attached to the substrate 520. Once the piezoelectric plate 510 is separated, the separated surface can be further polished via a suitable polishing method to prepare the surface for IDT formation, for example by chemical-mechanical polishing (CMP).

A conductor pattern including IDTs 530 is then formed on the separated surface, as shown in FIG. 5E. The IDTs are formed on the polished surface as described above. The photomasks used to define the conductor pattern are aligned to the alignment pattern. Each IDT 530 can be positioned on the separated surface of the piezoelectric plate 510 opposite a cavity 530. Final processing of the XBAR 500 then proceed with no further etching required.

FIG. 6 is an example schematic circuit diagram and layout for a high frequency band-pass filter 600 using XBARs. The filter 600 has a ladder filter architecture including three series resonators 610A, 610B, 610C and two shunt resonators 620A, 620B. The three series resonators 610A, 610B, and 610C are connected in series between a first port and a second port. In FIG. 6, the first and second ports are labeled “In” and “Out”, respectively. However, the filter 600 is symmetrical and either port and serve as the input or output of the filter. The two shunt resonators 620A, 620B are connected from nodes between the series resonators to ground. All the shunt resonators and series resonators are XBARs.

The three series resonators 610A, B, C and the two shunt resonators 620A, B of the filter 600 are formed on a single plate 630 of piezoelectric material bonded to a silicon substrate (not visible). Each resonator includes a respective IDT (not shown), with at least the fingers of the IDT disposed over a cavity in the substrate. In this and similar contexts, the term “respective” means “relating things each to each”, which is to say with a one-to-one correspondence. In FIG. 6, the cavities are illustrated schematically as the dashed rectangles (such as the rectangle 635). In this example, each IDT is disposed over a respective cavity. In other filters, the IDTs of two or more resonators may be disposed over a single cavity.

Description of Methods

FIG. 7 is a simplified flow chart showing a process 700 for making an XBAR or a filter incorporating XBARs. The process 700 starts at 705 with a substrate and a slab of piezoelectric material and ends at 795 with a completed XBAR or filter. The flow chart of FIG. 7 includes only major process steps. Various conventional process steps (e.g. surface preparation, cleaning, inspection, baking, annealing, monitoring, testing, etc.) may be performed before, between, after, and during the steps shown in FIG. 7.

The piezoelectric plate may be, for example, Z-cut lithium niobate or lithium tantalate as used in the previously presented examples. The piezoelectric plate may be some other material and/or employ some other cut angle. The substrate may preferably be silicon. The substrate may be some other material that allows formation of cavities by etching or other processing.

One or more cavities are formed in the substrate at 710, before the piezoelectric slab is bonded to the substrate at 720. A separate cavity may be formed for each resonator in a filter device. The one or more cavities may be formed using conventional photolithographic and etching techniques. Typically, the cavities formed at 710 will not penetrate through the substrate, and the resulting resonator devices will have a cross-section as shown in FIG. 3A or FIG. 3B.

An alignment pattern can also be formed in the substrate. The alignment pattern may be formed at the same time as the cavities, or before or after the formation of the cavities. Photomasks used in further steps can be aligned with the substrate via the alignment pattern.

Optionally, the substrate can be also be coated with SiO2 prior to being bonded to the piezoelectric slab.

At 720, a back-side dielectric layer may be formed on the piezoelectric slab. If used, the back-side dielectric layer may be formed by depositing one or more layers of dielectric material on the back side of the piezoelectric slab. The one or more dielectric layers may be deposited using a conventional deposition technique such as sputtering, evaporation, or chemical vapor deposition. The one or more dielectric layers may be deposited over the entire surface of the piezoelectric slab. Alternatively, one or more lithography processes (using photomasks) may be used to limit the deposition of the dielectric layers to selected areas of the piezoelectric slab, such as only opposite where the interleaved fingers of the IDTs will be located. Masks may also be used to allow deposition of different thicknesses of dielectric materials on different portions of the piezoelectric slab.

At 730, the piezoelectric slab is bonded to the substrate. The piezoelectric slab and the substrate may be bonded by a wafer bonding process. Typically, the mating surfaces of the substrate and the piezoelectric slab are highly polished. One or more layers of intermediate materials, such as an oxide or metal, may be formed or deposited on the mating surface of one or both of the piezoelectric slab and the substrate. One or both mating surfaces may be activated using, for example, a plasma process. The mating surfaces may then be pressed together with considerable force to establish molecular bonds between the piezoelectric slab and the substrate or intermediate material layers. The piezoelectric slab may be aligned to the alignment pattern of the substrate.

At 740, a piezoelectric plate is separated from the piezoelectric slab, e.g., via ion beam wafer slicing. The newly exposed surface of the piezoelectric plate opposite the substrate can then be polished in preparation for formation of a conductor pattern.

A conductor pattern, including IDTs of each XBAR, is formed at 750 by depositing and patterning one or more conductor layer on the front side of the piezoelectric plate, where the IDTs are aligned with cavities on the opposite side of the piezoelectric plate. The conductor layer may be, for example, aluminum, an aluminum alloy, copper, a copper alloy, or some other conductive metal. Optionally, one or more layers of other materials may be disposed below (i.e. between the conductor layer and the piezoelectric plate) and/or on top of the conductor layer. For example, a thin film of titanium, chrome, or other metal may be used to improve the adhesion between the conductor layer and the piezoelectric plate. A conduction enhancement layer of gold, aluminum, copper or other higher conductivity metal may be formed over portions of the conductor pattern (for example the IDT bus bars and interconnections between the IDTs).

The conductor pattern may be formed at 750 by depositing the conductor layer and, optionally, one or more other metal layers in sequence over the surface of the piezoelectric plate. The excess metal may then be removed by etching through patterned photoresist. The conductor layer can be etched, for example, by plasma etching, reactive ion etching, wet chemical etching, and other etching techniques.

Alternatively, the conductor pattern may be formed at 750 using a lift-off process. Photoresist may be deposited over the piezoelectric plate and patterned to define the conductor pattern. The conductor layer and, optionally, one or more other layers may be deposited in sequence over the surface of the piezoelectric plate. The photoresist may then be removed, which removes the excess material, leaving the conductor pattern.

At 760, a front-side dielectric layer may be formed by depositing one or more layers of dielectric material on the front side of the piezoelectric plate. The one or more dielectric layers may be deposited using a conventional deposition technique such as sputtering, evaporation, or chemical vapor deposition. The one or more dielectric layers may be deposited over the entire surface of the piezoelectric plate, including on top of the conductor pattern. Alternatively, one or more lithography processes (using photomasks) may be used to limit the deposition of the dielectric layers to selected areas of the piezoelectric plate, such as only between the interleaved fingers of the IDTs. Masks may also be used to allow deposition of different thicknesses of dielectric materials on different portions of the piezoelectric plate.

In all variations of the process 700, the filter device is completed at 770. Actions that may occur at 770 include depositing an encapsulation/passivation layer such as SiO₂ or Si₃O₄ over all or a portion of the device; forming bonding pads or solder bumps or other means for making connection between the device and external circuitry; excising individual devices from a wafer containing multiple devices; other packaging steps; and testing. Another action that may occur at 770 is to tune the resonant frequencies of the resonators within the device by adding or removing metal or dielectric material from the front side of the device. After the filter device is completed, the process ends at 795.

Closing Comments

Throughout this description, the embodiments and examples shown should be considered as exemplars, rather than limitations on the apparatus and procedures disclosed or claimed. Although many of the examples presented herein involve specific combinations of method acts or system elements, it should be understood that those acts and those elements may be combined in other ways to accomplish the same objectives. With regard to flowcharts, additional and fewer steps may be taken, and the steps as shown may be combined or further refined to achieve the methods described herein. Acts, elements and features discussed only in connection with one embodiment are not intended to be excluded from a similar role in other embodiments.

As used herein, “plurality” means two or more. As used herein, a “set” of items may include one or more of such items. As used herein, whether in the written description or the claims, the terms “comprising”, “including”, “carrying”, “having”, “containing”, “involving”, and the like are to be understood to be open-ended, i.e., to mean including but not limited to. Only the transitional phrases “consisting of” and “consisting essentially of”, respectively, are closed or semi-closed transitional phrases with respect to claims. Use of ordinal terms such as “first”, “second”, “third”, etc., in the claims to modify a claim element does not by itself connote any priority, precedence, or order of one claim element over another or the temporal order in which acts of a method are performed, but are used merely as labels to distinguish one claim element having a certain name from another element having a same name (but for use of the ordinal term) to distinguish the claim elements. As used herein, “and/or” means that the listed items are alternatives, but the alternatives also include any combination of the listed items. 

It is claimed:
 1. An acoustic resonator device comprising: a substrate comprising a cavity and an alignment pattern in a surface of the substrate, wherein the cavity and the alignment pattern have a same depth; a piezoelectric plate having front and back surfaces, the back surface attached to the surface of the substrate, wherein a portion of the piezoelectric plate that spans the cavity forms a diaphragm; and an interdigital transducer (IDT) on the front surface of the piezoelectric plate, wherein interleaved fingers of the IDT are on the diaphragm.
 2. The device of claim 1, wherein the IDT and the piezoelectric plate are configured such that a radio frequency signal applied to the IDT excites a primary acoustic mode in the diaphragm.
 3. The device of claim 1, wherein the alignment pattern is a trench.
 4. The device of claim 1, wherein the alignment pattern is configured to facilitate alignment of the piezoelectric plate and the interleaved fingers.
 5. The device of claim 1, wherein the substrate comprises Si.
 6. The device of claim 6, wherein the substrate further comprises an SiO₂ coating.
 7. The device of claim 1 further comprising at least one of a front-side dielectric layer on the front surface of the piezoelectric plate and a back-side dielectric layer on the back surface of the piezoelectric plate.
 8. A filter device comprising: a substrate comprising a plurality of cavities and an alignment pattern in a surface of the substrate, wherein the plurality of cavities and the alignment pattern have a same depth; a plate having front and back surfaces, the back surface attached to the surface of the substrate, wherein portions of the piezoelectric plate that span each of the plurality of cavities form respective diaphragms; and a conductor pattern formed on the front surface, the conductor pattern comprising a plurality of interdigital transducers (IDTs) of a respective plurality of acoustic resonators, wherein interleaved fingers of each of the plurality of IDTs are on respective diaphragms.
 9. The device of claim 8, wherein each of the plurality of IDTs and the piezoelectric plate are configured such that radio frequency signals applied to respective IDTs excite primary acoustic modes in the respective diaphragms.
 10. The device of claim 8, wherein the alignment pattern is a trench.
 11. The device of claim 8, wherein the alignment pattern is configured to facilitate alignment of the piezoelectric plate and the interleaved fingers
 12. The device of claim 8, wherein the substrate comprises Si.
 13. The device of claim 12, wherein the substrate further comprises an SiO₂ coating.
 14. The device of claim 8 further comprising at least one of a front-side dielectric layer on the front surface of the piezoelectric plate and a back-side dielectric layer on the back surface of the piezoelectric plate.
 15. A method of forming an acoustic resonator device, the method comprising: forming a cavity and an alignment pattern in a substrate, wherein the cavity and the alignment pattern have a same depth; attaching a back surface of a piezoelectric plate having front and back surfaces to the surface of the substrate, wherein a portion of the piezoelectric plate that spans the cavity forms a diaphragm; and forming an interdigital transducer (IDT) formed on the front surface of the piezoelectric plate, wherein interleaved fingers of the IDT are on the diaphragm.
 16. The method of claim 15, wherein the IDT and the piezoelectric plate are configured such that a radio frequency signal applied to the IDT excites a primary acoustic mode in the diaphragm.
 17. The method of claim 15, wherein the alignment pattern is a trench.
 18. The method of claim 15, wherein the alignment pattern is configured to facilitate alignment of the piezoelectric plate and the interleaved fingers.
 19. The method of claim 15, wherein the substrate comprises Si.
 20. The method of claim 19, wherein the substrate further comprises an SiO₂ coating. 